The present invention relates to a television receiver having means for suppressing the so-called line flicker.
It is an international standard that TV pictures are transmitted and reproduced according to the interlace principle, whereby a whole picture consists of a number of lines and is composed of two half pictures, one of which is formed by the even numbered lines of the whole picture, while the other half picture comprises or is provided by the intermediate uneven line numbers. The two half pictures are reproduced in an alternating manner by a fast, successive scanning of the respective lines. The lines, though disposed with small mutual spacing, are clearly recognizable on TV screens of usual sizes.
In the reproduction of pictures with soft light intensity transitions between different horizontal sub areas of the whole picture the alternating jumps between the two interlaced half pictures are visually acceptable, which is true also, generally for rapidly moving pictures, but in stillstanding and slowly moving pictures with sharp dark/light transitions clearly visible line jumps may be observed. In the two half pictures, the border line between the white/light area and the black/dark area will be reproduced with a mutual spacing of one line due to the alternating reproduction of the two half pictures. This is called line flicker, as the jumps as such may not be clearly visible, but rather manifest themselves as an unpleasant flicker. The whole picture frequency is 50 or 60 Hz, and the discussed jumps of the transition line between light and dark in the two half pictures will thus take place with a frequency as low as 25 or 30 Hz. It is a widely used standard that the whole picture holds a total of 625 lines, and the eyes of a viewer, who is placed at a normal distance from a reasonably large screen, may well perceive disturbances when such a transition or border line carries out jumps of approximately 1/625 of the height of the screen. Another applied standard comprises only 525 lines.
Methods have already been proposed for removing or making invisible the line flicker in an interlaced TV picture, but it has been found that the result is obtained at the expense of other kinds of disturbances of the picture, such that the problem really has just been pushed, and the associated techniques have even been expensive. According to one known solution use is made of a so-called progressive scanning, whereby the preceding half picture is stored by a memory and is reproduced together with the new half picture, such that the reproduced picture is a whole picture produced by a simultaneous reproduction in both line systems. Hereby a stationary horizontal border line will of course not carry out any jump, but as the memorized half picture is reproduced with a certain delay, the border lines between details of the picture may easily get blurred when there are movements in the picture, because the stored half picture all the time will show a movement phase belonging to the past. Thus, vertical border or separation lines which are moved horizontally will become jagged and therewith blurred, while a horizontal separation line which is moved vertically will carry out the movement in an unevenly jumping manner, which manifests itself as bluriness or muzziness. With another type of proposal the two last half pictures are stored in a memory, whereafter they are reproduced sequentially two times with a doubled frequency. Here the picture is still interlaced, but the frequency being doubled, also the frequency of all flicker will be doubled, whereby the flicker becomes invisible. However, it gives rise to a certain muzziness, and moreover the problem will exist that the reproduction is effected in a manner partly distorted with respect to the time, whereby even movements of separation lines are percieved as shaky or muzzy.
Line flicker is particularly problematic in pictures with text or graphics, and for some types of such reproductions, for example so-called teletext or -graphics, it has already been suggested to solve the problem with the use of a correction signal that is supplied to the vertical deflector system so as to effect a displacement of one half picture into coincidence with the other half picture. Hereby the resulting number of lines will be halved, i.e. the lines will be more clearly visible, but this may be visually acceptable for just that kind of reproductions, particularly when the line flicker is suppressed. The correction signal is derivable from a blanking signal which marks the cease of the original video signal and the start of the teletext signal. However, such a technique, which is described in DE-A-30 38 144 and 34 41 905, is not practically usable in connection with ordinary video pictures, where the vertical picture resolution would be unacceptably reduced.
It is the purpose of the invention to provide a TV receiver having means for removing the line flicker in a simple and efficient manner and without disturbing the correct succession of the half pictures, i.e. without the drawbacks associated therewith, and without giving rise to other considerable deficiencies of the reproduction.
The invention is based on the recognition that the last mentioned prior proposal may be further developed so as to be successfully usable with ordinary video pictures, for example by causing the line track on the screen to be locally displaced at such and only such place or places where dark/light transitions occur in the vertical direction, such that these transition places in the respective two half pictures will be brought closer to each other or be brought to fall together without the remaining picture details being treated in the same manner. The control signal to bring about such a displacement of a line or line portion in one or both half pictures is produceable based on a detection of the occurrence of the said abrupt dark/light transitions by detector means as described below.
Hereby the line flicker can be counteracted purely locally, by a local displacement of a line or a portion of a line in one or both half pictures, and it is thus not necessary to give up the use of the interlacing principle nor the successive, time true reproduction of the half pictures, whereby a first result is that with this solution of the problem it is at least ensured that the solution is not transformed into new problems elsewhere on the screen. As explained below it has been found, moreover, that the solution according to the invention is usable also without giving rise to perceivable distortions of the reproduction at the critical places themselves, and that even additional advantages have been observed, i.e. the invention may be realized in a manner such that the problem is solved practically without disadvantageous side effects.
According to the invention there is accordingly provided a television receiver having means for suppression of line flicker, such means comprising a detector unit for detecting a potential line flicker producing operation of the receiver and a control unit, which, in response to such a detection, is operable to produce a correction signal to the vertical deflector system of the picture reproducing unit of the receiver, by which the line track of the picture spot is disposed upwardly or downwardly to a location near or nearer the line level of the corresponding neighboring line in the preceding half picture. The detector unit is designed and arranged for continuous detection of the difference between the intensity of the actual video signal and the intensity of the video signal reproduction in a corresponding spot in at least one overlying or underlying line in an interlace pattern on the screen, and the control unit is operable to produce the control signal only in such situations in which it is detected by the detector unit that the picture spot is moved along a line portion forming a horizontal line of separation between over- and underlying picture areas of pronounced mutually different light intensity.
The local deformation or displacement of a line or a line portion on the screen of a traditional TV receiver is easily achievable by a modulation of the current to the vertical deflector coil for the cathode ray beam, which is sent against the inner side of the screen for describing or providing the lines by an associated horizontal deflection, or, respectively, by feeding a current to an additional vertical deflector coil, which will thereby weaken or strengthen the effect of the ordinary deflector coil, when corrections are to be made. When a detection of the light contents of some successive lines has revealed a jump of the light intensity, such a detection should be used for producing a change of the vertical deflection to the effect that a compression takes place towards the light area, since by a displacement of a line a dark area will be left, which should naturally be associated with the already dark area. It is achievable hereby that a separation line in one of the half pictures can be displaced to coincide exactly with the corresponding separation line in the other half picture, such that the line flicked will be totally eliminated.
What is left is that a slightly increased contrast between the light and the dark areas on the respective sides of the separation line will occur, because the light border area will be still lighter by the contribution of the corrected half picture, while the correction itself will induce an increased darkness in the dark area immediately adjacent the border line; however, experiments have proved, that this is in no way any disadvantage, but rather a direct advantage for a good picture reproduction. The reproduction of sharp transitions is improved considerably without any associated reduction of the reproduction quality of the less sharp transitions and without any problems with respect to panned or diffusely moving pictures.
For detecting the light contents of some previous and some following lines it is necessary to make use of a delay unit that will register the light contents prior to the lines being reproduced on the screen, and since some of these lines belong to the previous half picture, it is also with the invention required to employ a memory for these corresponding lines of the preceding half picture. However, the information to be memorized only has to relate to the respective points being pronounced dark or light, as the information should not be used for a renewed reproduction of the half picture, and information of such a type may be stored by a relatively simple memory unit.